LDR Reversal

[ScuzZ]

LDR -> More resistance with less light.

How would one, create a circuit, without an IC's, to make the LDR work the opposite way?

When light is present on the LDR for an output device eg. LED to go off, but when light is not present the output device goes on.?

 

[philba]

how are you going to drive the LED? via a transistor? just use two stages. or use a divider for the transistor input. where you place the LDR in the divider determines how it turns on the transistor.

 

[Sig239]

You can google for "dark activated alarm" or "dark activated LED" also.

 

[Gayan Soyza]

LDR -> More resistance with less light.

How would one, create a circuit, without an IC's, to make the LDR work the opposite way?

When light is present on the LDR for an output device eg. LED to go off, but when light is not present the output device goes on.?

Here is a simple method with descretes working pretty good.change the position
of the LDR & the V/R to get work in the other side.

 

[Nigel Goodwin]

Simple, yes - but it's also really poor, you can make a FAR better one with a simple opamp wired as a comparator which will work properly.

 

[philba]

the OP didn't want to use an IC. I find that kind of silly but that was his requirement.

 

[ScuzZ]

The voltage divider circuit does work, but what if you were attempting to set the amount of lumens to set the led off?

say around 500 lumens, in that circuit, the led would not light completely.

 

[Nigel Goodwin]

Like I said, it's a VERY poor circuit, and won't work at all well - an opamp will make a proper circuit, or you could improve it by adding a second transistor and making a schnitt trigger.

 

[Gayan Soyza]

The voltage divider circuit does work, but what if you were attempting to set the amount of lumens to set the led off?

say around 500 lumens, in that circuit, the led would not light completely.

Cant you do it by yourself ? Its the first circuit in college basic electronics.what's the transistor & supply voltage you use? You dont like to add an IC for this isn't it? even you can add a another transistor for the circuit & check.do it yourself....

 

[ScuzZ]

I never did electronics at school.

Electronics is a new field for me, the reason why I ask is because I don't know. And nothing I find on any search engine comes close to the information provided by those in the know.

I don't use IC's in any circuits, because I don't currently understand them completely. I work with what I understand.

At the moment, I have a limited supply of components, none of which are IC's, a few simulating programs and a breadboard.

With the help of others I've been teaching myself.

 

[ikalogic]

if i understand what you meant...

 

[Hero999]

You don't need an IC, you can make a smitt trigger from discrete components.

**broken link removed**

 

[Roff]

Hero, you need to add a current-limiting resistor between the collector of the NPN and the base of the PNP.

 

[Nigel Goodwin]

Very true - or the top transistor will blow - it's also more usual to use two NPN transistors with the emitters connected together for a schmitt trigger.

 

[Hero999]

I found that circuit from Google.

Yes, I didn't bother checking it.

Here's the circuit I normally use for this kind of thing, you can replace the PNP transistor with a p-channel MOSFET and 1M pull-up resistor and it can switch larger loads without requiring a relay.

 

[Nigel Goodwin]

Even Wikipedia gives the standard two transistor scmitt circuit.
https://en.wikipedia.org/wiki/Schmitt_trigger